Feeding Magnesium to Horses

There has been a lot of discussion about magnesium supplementation in horses. It has been reported to have a calming effect as well as being beneficial for obese horses and those predisposed to grass induced laminitis. However, before you go out and purchase a magnesium supplement for your horse we need to understand how magnesium works in the equine body and the potential problems can arise if too much is fed.

What is Magnesium all about?

Magnesium constitutes about 0.05% of the body mass. Sixty percent of magnesium in the body is found in the skeleton and about 30% in the muscle. Magnesium is important in the blood and plays a role as an activator of many enzymes and participates in muscle contractions. Magnesium deficiencies or abnormalities have an effect on neuromuscular function and cardiac tissue. As well as being important for the function of nerve and muscle, Magnesium is involved in the formation of one of the principle components of bone. In addition, Magnesium is necessary for the maintenance of electrolyte balance, particularly for Calcium and Potassium. Magnesium is also a very important as a co-factor in enzymes. Magnesium is primarily absorbed from the small intestine.

Recommended Daily Intake

The normal blood level for Magnesium in horses is 2.2-2.7mg/dl, according to National Research Council in 2007 (NRC), serum magnesium values below 1.6mg/dl are defined as hypomagnesaemia. According to NRC, an intake of 20mg of Magnesium per kilogram of bodyweight per day is necessary to maintain normal blood serum levels. Thus, for a 500kg horse in light to moderate exercise, an intake of 10g per day is necessary to maintain blood levels at the minimum value reported.

Magnesium Deficiencies and Excesses

Deficiencies in magnesium can result in nervousness, muscle tremors, incoordination, increased respiration and even death. Many commonly used feed ingredients (e.g. Lucerne, beet pulp) contain about 0.1 to 0.3 percent magnesium with an absorption rate of approximately 40 to 60 %. Excessive magnesium will be excreted in the urine, but overdoses have been linked to decreased calcium and phosphorus uptake, compromised intestinal integrity, heart conduction problems and renal trouble, so it’s important not to over supplement. The maximum tolerable level in the total diet is estimated at 0.8%, for a 500kg horse in light to moderate exercise consuming 2 to 2.25% BW that equates to 80 to 90g of magnesium in the total diet.
Magnesium sulfate intravenously injected directly affects the heart, forcing it into a state of arrhythmia. When used in larger doses, it can shut down cardiac function, result in subsequent collapse of the horse, and in some cases, death may occur. This practice should never be done.

Magnesium Sources

The most common form of Magnesium available is Magnesium Sulfate, commonly called Epsom salts, however one side effect of regular feeding of Epsom salts is diarrhea. The usual form used in animal feeds is Magnesium Oxide, a fine white powder. Magnesium Oxide is about 50% absorbable, depending on the relative levels of Calcium also present. The advantage of magnesium oxide is that the body will not absorb it if there is no deficiency, so it is difficult to overdose a horse using this source.

Equine Research and Magnesium

Magnesium is often fed as a calming supplement and until recently this was an anecdotal practice. A group of researchers in Australia fed 10g of added magnesium aspartate (total diet added mg plus ration – hay and concentrate, was approx. 21g of magnesium) and compared the flight response to horses that had been administered 0.04mg/kg BW of Acepromazine* and noted similar responses. This was the first study to compare magnesium supplementation to a known sedative agent. It should be noted that urinary calcium excretion was also higher in those horses supplemented with 10g of magnesium. This was the first study to compare magnesium supplementation to a known sedative agent. No more than 30g of magnesium should be in the total diet as a safe upper limit.

Magnesium may also play a role in insulin resistance and other disorders associated with equine metabolic syndrome. Magnesium deficiency has been associated with insulin resistance and metabolic syndrome in rodents and humans, but has yet to be scientifically confirmed in the horse. Although there have been anecdotal reports in horses of supplemental magnesium improving insulin resistance, an equine study conducted at the University of Tennessee have found no advantage to feeding a magnesium supplement to insulin-resistant, laminitic horses. This is potentially due to equine diets not being deficient in magnesium.

All of the HYGAIN concentrated or full feeds fed with hay or pasture meet the magnesium requirements for horses set by the Nutrient Requirements Council in 2007.

*Acepromazine or acetylpromazine (more commonly known as ACP, Ace) is a phenothiazine derivative antipsychotic drug. Acepromazine is frequently used in animals as a sedative, its principal value is as a chemical restraint in hyper or fractious animals.